The present invention relates to fasteners in general, and, more in particular, to panel fasteners.
Panel fasteners find application in many environments. One such environment is in aircraft where panels of the aircraft must be periodically removed from substructures to gain access to interior spaces.
Panel fasteners in the art include one type that employs a stud assembly mounted to a subpanel, understructure, or substructure through a basket, which assembly includes an axially extending, male threaded stud held in place by a barrel within the barrel. The stud points toward the understructure, but terminates behind it. A stud nut of the panel fastener has a head for bearing on a top panel and internal threads for engaging the threads of the stud. A retaining ring in a groove in the stud nut can retain the stud nut with the top panel. The stud nut tightens onto the threads of the stud to clamp the top panel and subpanel together.
In high performance and supersonic aircraft the skin of the aircraft carries load. Panel fasteners used with removable panels of that skin are required to have substantial structural strength. Because the skin carries load, when a panel is removed stresses move the understructure. This results in shifts in the hole pattern in the understructure. The movement occurs progressively with the detachment of more and more panel fasteners of a panel with the last remaining panel fasteners of a panel substantially loaded in shear making their removal difficult. Reinstallation requires a progressive shifting back of the hole pattern to the mated fit between the panel and the understructure. Thus the panel fastener must be removable and installable over a reasonable range of deflection between a substructure and a panel in which the nut of the fastener is attached.
This deflection between the substructure and the panel occurs both axially of the panel fastener, by way of gaps between the panel and the substructure, and radially of the panel fastener by way of shifting of the panel over the substructure in the plane of the panel. Gap closures of a magnitude of 0.125 inches may be required. Hole misalignment of a magnitude of 0.040 inches may have to be accommodated.
Because of the gap misalignment that must be accommodated, the panel fastener must have a fairly long stud nut to reach the stud before gap closure can occur. Lateral misalignment requires that the stud nut draw completely within the top panel to avoid catching on the substructure.
In the past it has been the practice to provide a retaining ring which functions to retain the stud nut with the top panel. When the stud nut retracts, the retaining ring snaps into an external groove on the stud nut to prevent withdrawal of the stud nut from the panel by interference of the retaining ring with the panel. The stud nut with this arrangement can freely move with respect to the top panel towards the substructure. This makes proximating the stud nut with the stud in the substructure difficult when the top panel lies below the substructure because the stud nut by its own weight wants to go into the fully retracted position away from the stud.
The retaining ring in this commercial version of a panel fastener also may be exposed when the stud nut moves from within the top panel towards the substructure. The retaining ring can come off the stud nut losing its function and permitting the stud to separate from the top panel.
Another problem attends exposed lock rings. This problem results from the inevitable use of gasket material on the panels. Gasket material can contact the lock ring when the lock ring is in its open position and prevent the lock ring from closing. Closure is necessary to permit full withdrawal of the stud nut into the top panel for and removal of the top panel from the substructure when some slip must occur.
U.S. Pat. No. 3,765,465 to Bulent Gulistan, the inventor of the subject matter of the present specification, discloses a panel fastener that employs a grommet to retain the retaining ring. The grommet has a head for bearing on the exposed side of the top panel and an axially extending portion that is flared over the bottom of the top panel to secure the grommet in place. With this arrangement the retaining ring is captive and the problem of its loss when the extension of the stud nut is overcome. Gasket material, however, can still reach the region where the lock ring is present and interfere with the proper function of that ring as by locking it open, because no barrier exists to stop the gasket material. The required flare at the bottom of the grommet is not thick enough in the axial direction to provide a barrier. Further, gasket material can enter the barrel and clog an annular space between the stud and barrel wall to prevent satisfactory operation of the cooperating stud nut. In a serious case, the stud nut would not be able to fully seat on the grommet and top panel because of the interfering gasket material.
The extent of protrusion of a panel fastener within the interior space behind the substructure or panel must of necessity be as small as possible because of weight and space considerations, especially in high performance aircraft. Constraints as to protrusions include the requirement of recessed heads in panel fasteners and of the provision of an axial space for wrenching the stud nut. A second constraint includes the requirement of having sufficient numbers of threads for effective engagement between the stud nut and the stud. Additionally, panel fasteners are used over and over again. Problems have been experienced in the past with thread locks losing effectiveness because of wear of the threads deformed to produce the lock. A wrenching recess in the head of the nut can strip with repeated use.